Plain tex margin note solution pulls text on preceding lines down (probably by the depth of strut)

Question

For the plain tex margin note solution suggest by 'wipet' here, it seems like all the lines preceding, and including the line at which margin note is positioned are pulled down by some length (probably depth of strut).

This screenshot shows how my compiled document looks like (see how first 3 lines are shifted down towards the following lines):

What follows is my complete example where I place this note in different settings on 3 new pages. Command to run this is >>lualatex marginnote.tex. You can uncomment the line that declares package lua-visual-debug to add debug markers too.

% marginnote.tex

\documentclass[notitlepage,letterpaper]{article}

% NOTE: Uncomment the following line, and run with lualatex if you want to visualize debug markers
%\usepackage{lua-visual-debug}
\usepackage{geometry}
\usepackage{fontspec}
\usepackage{blindtext}
\geometry{left=2in,right=2in,top=1in,bottom=1in}
\setmainfont{Verdana}

% wipet's original plain tex margin note solution::
% NOTE: Commented this as using \mnotefont gives me error (I tried with lualatex)
%\let\mnotefont=\tenit
% NOTE: Also increased the horizontal kern to 3em, for some reason 1em makes margin note
% collide with the main contents
\def\mnote#1{\strut\vadjust{\kern-\dp\strutbox\mnoteA{#1}\kern\dp\strutbox}}
\def\mnoteA#1{\smash{\llap{\hbox{\vtop{\mnoteB#1}\kern3em}}}}
\def\mnoteB{\hsize=1.7in \parindent=0pt \leftskip=0pt plus1fill}% \mnotefont}
% NOTE: Commented as doesn't seem relevant as I am using package geometry
%\hoffset=1.5in \advance\hsize by-\hoffset

\begin{document}

\thispagestyle{empty}

\newcommand{\mytesttext}{This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world.\mnote{This is a great world. This is a great world. This is a great world. This is a great world.} This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world. This is a great world.}

\mytesttext

\newpage
\thispagestyle{empty}



\begin{minipage}[t][1cm][t]{\linewidth}
    \mytesttext
\end{minipage}



\newpage
\thispagestyle{empty}

\newbox\myoddvbox
\setbox\myoddvbox=\vbox{{\hsize=\textwidth \mytesttext \endgraf}}%


\begin{minipage}[t][1cm][t]{\linewidth}
    \box\myoddvbox
\end{minipage}


\end{document}

Answer 1

The problem is that you used LaTeX and the macro is designed for plain TeX. It works without problems in plain TeX.

This is one of several situations where LaTeX defines original plain TeX macro differently. The macro \smash keeps vertical mode in plain TeX but it opens horizontal mode in LaTeX. Why? I don't know. See the line 5912 in latex.ltx:

\def\finsm@sh{\ht\z@\z@ \dp\z@\z@ \leavevmode@ifvmode\box\z@}

and the line 1040 in plain.tex:

\def\finsm@sh{\ht\z@\z@ \dp\z@\z@ \box\z@}

You can see the difference.

It is clear that my macro:

\def\mnote#1{\strut\vadjust{\kern-\dp\strutbox\mnoteA{#1}\kern\dp\strutbox}}
\def\mnoteA#1{\smash{\llap{\hbox{\vtop{\mnoteB#1}\kern3em}}}}

creates \kern\dp\strutbox in vertical mode in plain TeX (and this is correct) but it creates \kern\dp\strutbox in horizontal mode in LaTeX (and this is incorrect).

If we cannot rely on the fact that our macros will be used in plain TeX only (which is recommended) then we cannot use macros like \smash and we must to use only primitives instead such macros:

\def\mnote#1{\strut\vadjust{\kern-\dp\strutbox\mnoteA{#1}\kern\dp\strutbox}}
\def\mnoteA#1{\setbox0=\hbox to0pt{\hss\hbox{\vtop{\mnoteB#1}\kern3em}}%
              \ht0=0pt \dp0=0pt \box0 }